1. Field of the Invention
This invention relates to the cracking of hydrocarbons and more particularly to a method and apparatus which utilize fluidized catalytic cracking processes.
2. Description of the Prior Art
In a petroleum refining operation, large hydrocarbon molecules are cracked into smaller molecules for the production of motor fuels such as gasoline, jet fuel, kerosene and diesel fuel. This process is usually carried out in a fluidized catalytic cracker in which the catalyst in powdered or granular form can be effectively contacted with the heavy petroleum feedstock.
In a typical fluidized catalytic cracking process, the hydrocarbon feedstock and hot regenerated catalyst are injected into the base of an elongated riser. In some cases, fluidizing gas is used to increase the dispersion of the solids and improve the contacting of the feedstock and catalyst powder. The fluidized suspension passes upwardly through the riser where reaction occurs. The riser terminates in a reaction vessel where catalyst and hydrocarbon effluent are separated in a primary separation zone. The hydrocarbon passes through a cyclone separation device to remove the remaining particulate solid catalyst and then goes to product separation. The spent catalyst is collected in the base of the reaction vessel, stripped of residual hydrocarbon vapors with steam, and then passed to a regeneration section.
There are a number of means of making the primary separation of the hydrocarbon and solids in the reaction vessel. The simplest means is to simply exit into a vessel of sufficient diameter that the resultant gas velocity is insufficient to carry the solids which then fall to the bottom of the vessel where they are stripped of residual hydrocarbons. As reaction temperatures increase, there is a desire to reduce thermal reactions that continue even after the hydrocarbons are separated. Thus more rapid primary separation is desired. Many devices have been commercialized to affect a more rapid separation including rough cut cyclones, inverted "top hats", slotted risers, closed coupled cyclones, etc. The general characteristic of all of these devices is rapid separation and/or controlled effluent gas removal, possibly including quenching of the gases via the addition of various other cooler streams. These technologies are well known to those skilled in the art.
In the regeneration section, coke deposited during the reaction and any unstripped hydrocarbons are combusted with oxygen containing gases. The regeneration serves to reheat the solids and remove any residual coke deposits to restore catalytic activity. In general, the amount of combustible hydrocarbons that enter the regenerator are a function of the severity of the cracking reaction, the specific gravity and character of the feedstock, and the circulation rate of solids. The cracking severity defines the amount of coke deposited. Heavier and/or more aromatic feedstocks tend to deposit more coke at a given reaction severity. Higher solids circulation rates tend to carry more unstripped hydrocarbons into the regeneration zone. Not only do these hydrocarbons represent fuel ("circulation coke") but given the higher hydrogen content of the unstripped hydrocarbons, their heating value is greater than deposited coke. This leads to overheating of the solids and possible thermal deactivation.
There are many variations of regeneration systems for catalytic cracking. In some cases, a single stage combustion is used. In others, variations in contacting zones and or fluid dynamic conditions are used to provide specific benefits such as reducing peak temperatures during combustion, improve air/catalyst contacting, reducing net heat release to the solids, etc. In other variations, two separate combustion zones are used with separate air contacting in each. These are known to those skilled in the art and a few examples are U.S. Pat. No. 2,852,443, U.S. Pat. No. 3,909,392, U.S. Pat. No. 3,919,115.
Following the regeneration, the reheated solids are stripped of combustion products prior to being recycled to the riser reactor. Hydrocarbon feedstock is introduced into the base of the riser. Many different nozzle injection systems are used in commercial practice. The reaction proceeds as the fluidized mixture flows through the riser. The riser geometry sets the system residence time.
A fluidized catalytic cracking process operates in heat balance. The heat required for the endothermic heat of reaction is supplied by the fuel (coke and/or unstripped hydrocarbons) that flows to the regeneration section from the reaction section. If the fuel is insufficient for the desired conversion, the regeneration temperature will drop and the system will gradually reduce conversion to where the fuel equals the demand in the reactor. Conversely, if the fuel from the reactor is excessive, the catalyst will return to the reaction section incompletely regenerated (still fouled). The coke deposits on the catalyst cover active sites and thus effectively reduce the catalytic activity of the solids. In this case, conversions will fall until the system again reaches heat balance.
The principal desired products from a fluidized catalytic cracking process are diesel oils, gasolines, and C3 to C5 compounds, particularly isoparaffins and isoolefins as opposed to normal paraffins and olefins. Heavy fuel oils and light gases have value principally as low cost fuels and thus do not add appreciable value to the process.
The total reaction in any fluidized catalytic cracking reactor is a summation of thermal and catalytic reactions. Thermal reactions are driven by temperature. The products of thermal reactions contain high percentages of less valuable C2 and lighter compounds by the very nature of the cracking kinetics. Thermal reactions proceed whether or not solids are present and are suppressed only by lowering the temperatures of the reaction.
Catalytic reactions on the other hand are driven by a combination of temperature, the number of catalytic sites involved in the reaction and the activity of each individual site. The products of the catalytic reactions are principally diesel oils, gasolines, and C3 to C5 compounds. Further, the C3 to C5 compounds formed have a high percentage of desired iso compounds due to the inherent isomerization activity of the typical zeolitic acidic cracking catalysts.
Increasing the catalyst to oil ratio in the process will increase the catalytic conversion at constant temperature while the extent of the thermal reactions will remain the same. Thus high catalyst to oil cracking will result in a higher conversion at any given temperature with the increase being due to catalytic reactions. Thus the effluent yield will show a higher percentage of total products due to the catalytic reactions.
In order to maximize the production of gasolines and olefins, high conversions of feedstock are desired. In order to achieve high conversions, operators of fluidized catalytic cracking units have attempted to increase both catalyst to oil (C/O) ratios and operating temperatures. There are however, limits to the extent that this can be done in single riser units. Higher temperatures will result in higher thermal products which negatively affect economics. Higher C/O ratios will increase conversion at constant temperature but will bring increased quantities of unstripped hydrocarbons into the regeneration zone. In fact the quantity of unstripped hydrocarbons is proportional to the solids circulation rate. This will result in more fuel to the regenerator and higher solids temperatures. Higher solids temperatures will increase reaction outlet temperature at the higher circulation rates which leads to even higher light gas production. The only way to achieve high C/O ratio cracking in a conventional single riser system is to remove heat from the regenerator.
Two stage regeneration as described above is one means of reducing solids temperature at constant fuel. Alternately, heat removal via steam generation can be used. Both of these options are practiced commercially.
It is obvious from the above that the operator of a conventional fluidized catalytic cracking unit is limited in the ability to process a hydrocarbon feed at high catalytic conversions at low temperatures in order to both maximize the "catalytic content" of the yields (isomerization), achieve high feedstock conversions, and minimize the unwanted thermal products.
Operators are often faced with an additional problem. In a refinery there are typically a wide range of feedstocks that vary in specific gravity, boiling range, and composition. These will exhibit varying performance in a fluidized catalytic cracking reactor. It is well known that the lighter feedstocks (e.g. naphthas with boiling ranges from 38.degree.-204.degree. C.) require higher reaction severity in order to crack in comparison to vacuum gas oils for example. In order to process a number of feedstocks in a single unit, various processes have been developed to stage the feedstocks to the riser. This involves feeding the lighter, lower molecular weight portion of the feedstock which is more difficult to crack to the bottom of the riser and feeding the heavier, higher molecular weight portion to a higher point in the riser. In this regard, reference is made to U.S. Pat. Nos. 4,624,771, 4,435,279 and 3,186,805.
All of the above mentioned staged processes have a common feature. The effluent from the first feedstock contacting stage (lower portion of the riser) passes in its entirety to the second stage. Thus the feedstock feed to the first stage of the unit sees the entire residence time of the riser and the subsequent feeds see progressively shorter residence times as they are introduced higher and higher in the riser. Further, for a given catalyst circulation rate, the first feed sees the highest C/O ratio at the highest solids temperatures. It thus experiences the highest severity. Subsequent feedstocks however see progressively lower C/O ratios and lower temperatures as more feed is introduced and as the endothermic reactions reduce the reaction temperature. Further, each time the catalyst is contacted with a feedstock, fouling of the catalyst takes place. The extent of fouling depends upon the severity of the reaction (time and temperature) and the nature of the feedstock. Thus the last feed sees the lowest C/O, the lowest temperature, and a less active catalyst since reaction has been occurring up to that point. Operation of these types of staged systems leads to wide distributions in yields from each feed due to wide differences in reaction severity for the initial feed and final feed. The wide differences in conversions for the feeds leads to a non-optimal product yield spectrum consisting of some portions of overcracked and some portions of undercracked materials.
Another development in the field of fluidized catalytic cracking is represented by U.S. Pat. Nos. 4,925,632 and 4,999,100. These patents relate to what is referred to as a low profile fluid catalytic cracking process and apparatus wherein there is a succession of low profile catalyst chambers each containing a reservoir of catalyst and alternately connected in sequence by openings below the catalyst level and above the catalyst level. The catalyst in all chambers is fluidized by gas flowing upwardly through each chamber.
This process is a staged process that differs from the ones cited above. In this scheme, there are truly separate stages where hydrocarbon feedstock contacts catalyst and then is separated from that catalyst. The effluent gases are sent for further processing and the solids continue to the next stage where they contact a second feedstock.
The patents teach that a such a staged process will allow operation at a lower overall C/O ratio than a single riser system. The patent details a number of advantages all of which relate to operation at effectively lower catalyst circulation rates per unit of hydrocarbon processed. Lower circulation rates minimize the requirements for tall vessels to provide pressure for circulation. The lower circulation rates lead to lower fuel to the regenerator. The reduced circulation rates also reduce catalyst attrition and vessel erosion, both known to be a function of catalyst circulation. In addition, the lower vessels lend themselves to shorter residence times for reaction (shorter risers) which can improve yields.
The lower catalyst circulation rates are achieved by two means. First, the staged introduction of feeds with effluent separation between stages creates separate zones where a reduced net solids flow contacting only a portion of the feed results in a C/O ratio equivalent to a conventional unit but higher than that based upon total feed and catalyst flows. Secondly, the process utilizes common walls between reactors and regenerators to allow for indirect heat transfer from the hotter regeneration section to the reaction section. This minimizes the amount of solids circulation required to provide heat.